Cathode structures



Feb. 24, 1959 J. BEGGS CATHODE STRUCTURES Filed Oct. 22. 1954 e t 5 2 2W. 2 7 M ig g 18772532212 5 gM/M/ 15 115 Attorney.

United States Patent l 2,875,367 'ca'rnonn STRUCTURES James 'E. Beggsschenectady, N. Y., assignor to General Electric @ompany, a corporationof New York Application October 22, I954, SerialNo. 464,078 8 Claims.(Cl. 313-440 This invention relates to cathode structures and n1ethedsoftorming the same. While the cathode structures of this invention aresubject to many variations and modifications they are ideally suited forutilization in miniature electric discharge devices and will beparticularly described in this connection.

It is generally desirable that the electron emitting surface of anelectric discharge device be brought up to operating temperature at arapid rate and remain sub stantially stationary throughout the entirerangeof-operat-jing temperatures; Conductors having a relatively largesurface 'per unit volume can be rapidly heated and will transfer a largeportion of this heat at a rapid rate to another, member in contacttherewith. However, thin conductive members, such as foil, areundulyfiexible and tend to change their shape and position upon heating.It

is therefore an important-aspect of this ,inventionto protyide afastheating foil structure which is relatively rigid whilestill maintaininga relatively high rate of heat transferto-an electron emitting surface.

It'is also'desirable that the current supplied to the heating element besupplied in a symmetrical path so that the inherent inductance of thecurrent flowing through the heater leads is effectively cancelled a ndfurther so thatnall areas of the heated element are uniformly andrapidly heated. In addition, the cathode structure should have a lowinductance, particularly toradio frequency It ;-is therefore animportant object of tliis invention to .provide an improved. cathodestructure and method of According to an important aspect of thisinveiitiony .there is provided a1 cathode structure and method offorming a cathode structure which includes a foil member and an electronemitting surface oriented in heat conducting relati-onship'to. saidffoilmember toprovide a low inductance "unitormly and quickly heated cathodestructure.

" Other important aspects and obje'cts of this invention will becomemore apparent W'hen'the following specificatloii and claims are.consideredin connection with the figures of the drawing inwhichlFigurelillustrates-a U of sustained high or channel-shaped cathode formed inaccordance with this 1 I inventionyFigu'res 2 and? illustrate amodification of the cathode illiistrated assi ns l; Figure l illilstatesI ajca'tliode with a "foil heater elernent; "Figures saana illustrate af fb ll heater and-the adaptation M this; foil heater-to directly h atacup s haped cathode; Figure 6 il- 'lathe.

2,8?5,35Y Patented Feb. 244, 1955i ice 2 lustrates a cathode and heaterstructure and Figures 7 to 13, inclusive, illustrate modified cathodestructures wherein a conductive film is formed on an insulating memberto provide a heater.

Figure 1 illustrates a directly heated cathode assembly or thefilamentary type which is formed of a foil clad member consisting ofbacking 10 with foil covering 11. The cathode is formed, for example, bystamping the backed foil into the desired channel or U-shape and thenremoving backing metal 10, for example, by applying an acid bathWhiChCliSSOlVdS the backing metal. A portion of the backing metal 16 isleft at the ends of the cathode structure to assist in"attaching heaterleads and in mounting the cathode structure. An appropriateemissivecoating, such as the conventional triple carbonate coating,isthen applied to the top surface 12 after which the cathode 'assernblyis mounted in an electric discharge device and activated. s

For example, Where it isdesired to ton-n a cathode which is capable ofoperating at sustainedhigh temperatunes, the cathode structure is formedfromfa sheet of steel approximately 10 mils thick having a molybdenumtoil coating approximately .25 mil thick. The cathode is formedand thenthe steel is dissolved from the molybdenurn by applying a hydrochloricacid solution. In this manner, an extremely strong and rigid foilcathode structure isforrned without the attendanthazard ofcracking orbreaking the foil which is present when a' cathodeis formed fromunbackedfoih Acathode of this-type is particularly: suited forutilizationas a direct heatingcathodeand can be brought up to operatingtemp'erature in a very shor'tperiod of time, i. BL, in the orderdrone-quarter of a second.

It is readily apparent that this process and the form of cathode hereinillustrated, by way of example, issubject to a number of variations inshape and form and may be constructed of any foil material which forms asatisfactory cathode base and may, for example, consist of suchmaterials as iron, molybdenum, nickel, tungsten, titanium, zirconium ortantalum. 3 t

For example, an active barium oxide electron emitting surface can befabricated on a titanium base Without the formation of an interface, andwhich is capable of high level stableeinission at relatively low cathodetemperaturesu Theoxide'coating is readily reduced by the tita almost arate depending upon the cathode temperature, so that barium is formedand oxygen is "absorbed by the titanium without the formation of a filmor interfac'eon thesurface of thetitanium base and thereby re sults incathode emission which is stable at high current densities even'whenoperated in theemissionlimited region; The-emission lifeof aititaniumcathodecan be increased by utilizing one ornmorepof the oxides of."strontium or'calcium or combinations thereof with barium titanium moreslowly than is bariurnoxide. I g

' Figures 2 and 3 illustrate a' manner in which a.cup-

oxide, since strontium andcalcium oxides arereduced by shaped cathodestructure can be formed from a strip or ribbon of foil13 so as toprovideacathode structure consisting of afls'tibstantially flat top surface..14 havingcylindrical skirt members-15 and16 This cathodeyfor exampleisformed with a punch press or may be form'ed by spinning a strip orribbon of .foil on a It is apparent that this structure. can be formedfrom backed foil material from which the backing .metal is: removedafter forming.

The structure illustrated in Figure 3 hasthe additional feature ofhaving a substantially circular t'op surlface and substantiallycylindrical sides whichflendan addia .tionaludgr eepf rigidity to thefinal structnre- .The top rsurface 141is coated with an emissive coatingandthe slot 17,-which. is present when a .thin. strip on ribbon is soformed, or can be cut into a cup which is punched out of a flat disk orslug of metal, provides the two heater leads and results in aninherently non-inductive cathode structure.

This structure can be adapted for utilization as an indirect heater fora cathode structure such as that illustrated in Figure 4 wherein acathode structure '18 similar to that illustrated in Figure 3 issupported in heat conducting relationship with, but out of directcontact with inverted cup 19, on the top surface 20 of which is placed asuitable emissive coating. Cup 19 is supported, for example, in ceramicinsulator 21 which is provided with a plurality of projections 22 whichmake contact with and are bonded to the cathode. The projections limitthe heat transfer to the insulator and increase the efliciency of thecathode. The cathode connection is made by applying a conducting film tothe lower surface of insulator 22 so that radio frequency currents flowover the entire surface of cup 19 and result in a low inductance cathodeconnection.

Cathodes of the form illustrated in Figures 3 and 4 have inherently lowinductance so that they are ideally suited for utilization in miniatureelectric discharge devices operated at high frequencies. Since theelectrically heated cathode structure is formed of extremely thin foilmaterial, it can be very rapidly heated and in, view of the inherentrigidity of the shape into which it is formed, a cathode structureutilizing a heater element of this type is less subject to rnicrophonicsthan are other forms of heaters.

Figures and 6 illustrate modified versions of directly heated cathodestructures utilizing at least one conductive foil member having anelectron emitting surface .oriented in intimate heat conductingrelationship to the foil member. Figure 5a illustrates two substantiallyL-shaped foil strips orribbons 23 and 24 having angularly oriented topportions 23 and 24' which are rounded so as tofit into and be bonded tothe inner surface of inverted foil cup 25. The cup has a substantiallyfiat surface 26 on which an emissive coating is formed and the entirecathode structure is, for example, retained in metal disk 27 whichprovides an inherently low inductance conducting path to surface 26.

The cathode structure is formed by suitably bonding ribbons 23 and 24together along the length thereof and then bonding the heater to theinner surface of the cup 25. The bonding may be effected byspot-welding, brazing or other suitable metal-to-metal bonding such asis disclosed in my copending application Serial No. 409,159filed'February 9, 1954 and assigned to the same assignee as thisinvention.

The cup consists of metal, such as titanium, zirconium, iron-nickelalloy, tantalum or other metal, which forms a satisfactory cathode baseand, for example, is formed from foil or from backed foil. For example,the cup can be formed from titanium foil clad steel wherein the steel isdissolved in hydrochloric acid after the cup is formed or,alternatively, it may be fabricated by joining a shallow foil cup ordisk and a thin foil cylinder.

The ribbon heater is formed, for example, of a ribbon of titanium,zirconium, iron-nickel alloy, tantalum or other metal which can. besatisfactorily bonded to an inner surface of the cup 25. Thecrosssection of the Y ribbon heater is such that it heats. with approximatelythe samecurrent as will heat the cup to which it is attached so that theentire structure is uniformly and rapidly heated, i. e., the foil andthe cup form the heating element. Since the cup has relatively low heatconductivity,

as a result of the choice of material and its relative thinness, theheat loss from the sides of the cup is relatively small and theefiiciency of the resulting cathode structure is high.

An alternative form of a directly heated foil cathode is'illustrated inFigure '6 wherein foil cathode cup 28 is provided with a centrallyoriented heater lead 29 terminating in a tapered end portion 30 whichextends out to contact an area within cup 28 substantially the same asand contiguous with the emitting surface 31. The heater lead 29 andtapered end portion 30 have such dimensions that approximately the samecurrent heats the lead as will heat the cup 28 and so that the entirestructure is uniformly and rapidly heated.

It will be readily apparent that the cathode structures herein describedcan be modified, for example, by forming a convex or concave emittingsurface and, for example, by utilizing an electron emitter other thanthe aforementioned triple carbonate coating.

It is further noted that where a higher resistance heater is desired, aceramic button or refractory coating can be formed on the inner endsurface of cup 25. A conducting film can then be formed by applying acoating of carbon containing compound, such as a nitrocellulose binder,to the ceramic button and then applying at least one metal from thegroup consisting of titanium, zirconium, hafnium, and thorium and atleast one metal from the group consisting of, but not limited to, iron,

nickel, cobalt, copper, chromium, platinum and molybdenum so that whenheated a conducting film, including at least one metal from thesegroups, is formed. Conducting films of this type are more completelydescribed in my copending application Serial No. 464,080 filed herewithand assigned to the same assignee as this application.

Figure'7 illustrates a form of resistance heater utilizing the methoddisclosed and claimed in the above-identified patent application whereinthere is shown foil cup 32 having a top surface 33 on which emissivecoating 34 is applied. Within the cup a thin disk or wafer 35 ofinsulating material, such as a ceramic of the type commonly referred toas alumina, is suitably bonded to the top surface of, the cup. Aconducting heater film 36 is formed over a portion of or all of thelower surface of ceramic disk 35 and heater lead 37 connects to acentral portion of the conducting film 36. Heating is effected bycurrent flow through heater lead 37 to film 36 and down through thecup-shaped foil member 32.

It is noted that the foil cup can be punched or otherwise formed inaccordance with the previously described process or the cup can beformed by suitably bonding or brazing top member 33 to a cylinder offoil 32 at the same time the ceramic disk or wafer is bonded to the topmember. A method of effecting such a bond between ceramic and metalmembers is described in my aforementioned copending application, SerialNo. 409,159.

Alternatively, a coating of high temperature clay or other refractorymaterial can be substituted for the thin ceramic wafer. The surface iscoated with a conducting film towhich is brazed the center connectingheater lead 37. When voltage is applied, the conductive film becomeshot, thereby heating the electron emitting surface to which it isattached. The conductive film can be made to operate at voltages inttheorder of volts, in which case it can be operated from a plate batterysupply, and thereby eliminate the need for a separate heater supply IThe. rigidity of the insulating film or ceramic wafer ance heater 40formed on ceramic disk or wafer 41 and connected by heater leads 42 and43. Figures 9 through 13, inclusive, illustrate modifications of theheater pattern illustrated in Figure 8. For example, heaters illustratedin Figures9 throughll, inclusive, are particularly. suited a; vines?forutilization in separatelyheated cathode constructions a'n dthoseillustrated in Figures 12 and 13 are particularly suitedfor heaterswherein the cathodeycup or emitting surface forms a part of the heatercircuit. Each of these heater patterns is formed on a ceramic buttonor.disk of material suchas, for example, those ceramics generallyclassified as steatites, forsterites and aluminas.

It is noted that the ceramic button or disk 4 1 'can be covered withanitrocellulose coating over the entire area or in the form of thepattern illustrated in Figures 9 through 13, inclusive. When a highheater voltage is required, these films are usually of a type consistingessentially of carbon while, when low heater voltages are desired, thecoating includes metal additives so as to result in a conducting filmhaving a positive, zero, or negative thermal coefficient of resistance.Examples of such films and methods of forming the same are described ingreater detail in my aforesaid copending application Serial No. 464,080.

Cathodes of the type herein disclosed are ideally suited for utilizationin electric discharge devices such as those described in my Patent No.2,680,824 and my copending application Serial No. 464,079 filedherewith, both of which are assigned to the same assignee as thisinvention. T

It is readily apparent that the cathode structures of this inventionhave inherently low inductances and in particular those illustrated inFigures 5 through 8 provide a substantially symmetrical heater currentpath which has inherently low inductance. The metal ring, or. theceramic ring with a conducting coating thereon, which retains thecathode provides an inherently low inductance conducting path to theemissive surface which extends over the sides of the cathode. Theintimate heat conducting contact between the heater and the emittingsurface provides a structure which is rapidly heated to operatingtemperature, i. e., in the order of one quarter to three quarters of asecond, and which structure is relatively rigid because of the shapethereof so that the tend- .ency toward microphonics is minimized.

This invention has been described in connection with a limited number ofspecific examples so that a complete understanding thereof may be had;however, it will be readily apparent that this invention is subject tomany modifications and it is intended in the appended claims to coverall modifications coming within the true spirit and scope of thisinvention.

What I claim as new and desire to secure Patent of the United States is:

1. A cathode structure for an electric discharge device comprising acup-shaped member including side wall and an end wall having an electronemissive coating on by Letters an exterior surface thereof, a thininsulating disk in contact with the interior of said end wall and a thinfilm on the exposed surface of said disk and having a much greaterresistance than said cup-shaped member, said,

film extending to said side walls for providing a heater element inclosely coupled thermal relation with substantially all of said end walland an electrical connection with said film heater at apoint remote fromthe side walls of said cup'shaped member.

2. A cathode structure for an electric discharge device comprising acup-shaped member including an end wall having an electron emissivecoating on an exterior surface thereof, a thin insulatingdisk in contactwith the interior of said end wall, a resistance heater element bondedto the exposed surface of said insulating disk closely coupled thermalrelation with substantially all of said end wall, said resistanceelement having a much greater resistance than the resistance of saidcup-shaped prising a .thin metallic support, an electron emissivecoating onsaid support on one side thereof, a thin insulat- 6 ing waferin coma with the opposite side of said support andsubstantiallycoextensive in area with said coating, athin conductivefilm formed'on the opposite side of said wafer to provide a heater inclosely coupled ther mal relation with substantially all of the coatedarea of said support, said film having amuch: greater resistance thanthe resistance ofsaid support, an electrical connector connected withsaid conductive film at a central point thereof, and another electricalconnector connected to said conductive fihn at, at least, one point onthe periphery thereof.

4. A cathode for an electric discharge device comprising a thin metallicsupport, an electron emissive coating on said support on one sidethereof, a ceramic wafer in contact with the opposite side of saidsupport and substantially coextensive in area with said coating and athin metallized film formed on the opposite side of said ceramic waferin electrical contact with the periphery of said thin metallic supportaround the coated area to pro vide a heater in closely coupled thermalrelation with substantially all of the coated area of saidsupport, saidfilm having a much greater resistance than the resistance of saidsupport.

5. A cathode for an electric discharge device comprising a thin metallicsupport, an electron emissive coating on said support on one sidethereof, a thin ceramic Wafer in contact with the opposite side of saidsupport and substantially coextensive in area with said coating, and athin metallized film formed on the opposite side of said ceramic waferand in electrical contact with said thin metallic support at theperiphery of the coated 'area to provide a heater in closely coupledthermal relation with substantially all of the coated area of saidsupport, said film including a metal selected from the group consistingof titanium, zirconium, hafnium and thorium, said film having a muchgreater resistance than the resistance of said support.

6. A cathode for an electric discharge device co1nprising a thinmetallic support, an electron emissive coating on said support on oneside thereof, a thin ceramic wafer in contact with the opposite side ofsaid support and substantially coextensive in area with said coating,and a thin metallized film formed on the opposite side of said ceramicwafer and in electrical contact with said metallic support at theperiphery of the coated area to provide a heater in closely coupledthermal relation with substantially all of the coated area of saidsupport, said film including a first metal selected from the groupconsisting of titanium, zirconium, hafnium and thorium,

and a second metal selected from the group consisting of combinedresistance approximately equal to the resistance of said cup-shapedmember so that when a heating current is conducted through said ribbonsand said cup-shaped member said ribbonsand said cup-shaped member arerapidly and substantially uniformly heated.

8. A cathode structure for an electric discharge device comprising athin conducting cup-shaped member including an end wall having anelectron emissive coating on an exterior surface thereof, a conductorapproximately centrally oriented within said cup-shaped member, saidconductor having a taperedend portion extending out to.contactsubstantially all of said end wall, the resistivity of saidconductor being of the order of the resistivity of said cup-shapedmember and the cross-sectional area .of said conductor and saidcup-shaped member being such that upon passage of a heater currenttherethrough, said conductor and said cup-shaped member are rapidly andSubstantially uniformly heated.

References Cited in the file of tlris patent UNITED STATES PATENTS 11,859,522 Miessner May 24, 1932 Bi'uining et al. 'l Oct. 31, 1950

